Conventional electrical submersible well pumps for oil and deep water wells are supported on a string of production tubing. The production tubing comprises sections of steel pipe screwed together, each section being about thirty feet in length. The pump is a centrifugal pump driven by an AC motor located below the pump. A power cable extends from the surface alongside the tubing for supplying power to the motor. The power cable is strapped to the tubing at frequent intervals to support the weight of the power cable.
One disadvantage of the conventional pump assembly described above is that when the pump must be pulled for repair or replacement, the procedure is expensive. The operator needs a workover rig with the capability of pulling the sections of tubing. Pumps of this nature must be pulled typically at least every eighteen months. Considering the cost of the workover rig as well as the down time for the well, the periodic expense is significant.
A few installations have been made employing coiled tubing. Coiled tubing is a continuous string of metal tubing which is brought to the well site on a large reel. The coiled tubing unit unreels the tubing and forces it into the well. Coiled tubing has been used for various purposes in the past, and recently used to suspend electrical submersible pumps. An advantage of a coiled tubing supported pump is that it does not need a workover rig to pull it. Also, pulling and replacing it should be faster than production tubing.
One proposal in the past was to produce production fluid from the pump through the coiled tubing and strap the cable to the exterior of the coiled tubing. A disadvantage of this assembly is that a separate reel is needed for the power cable. Securing the straps would slow down the installation and pulling procedure. Furthermore, commercially available coiled tubing is not large enough in diameter for desired production in many cases.
Some installations have been made with the electrical cable installed within the coiled tubing. Production fluid from the pump flows through a casing surrounding the coiled tubing. The electrical cable is a three-phase cable having fairly large metal conductors. The weight of the cable is such that it will not support itself in a deep well. Even if inserted within coiled tubing, the weight of the electrical cable needs to be supported by the coiled tubing. In one type of installation, separate mechanical anchors are spaced along the length of the insulated electrical cable. The cable is inserted into the coiled tubing with the anchors retracted. The anchors are then shifted to a weight supporting position, gripping the inner diameter of the coiled tubing. U.S. Pat. 5,435,351, Head, Jul. 25, 1995, describes such a system.
Another proposal shown in U.S. Pat. 5,191,173, Sizer et al, Mar. 2, 1993, describes using an elastomeric jacket of a type that will swell when exposed to a hydrocarbon liquid. The jacket is extruded over the insulated conductors during manufacturing. The jacketed electrical cable is then inserted into the coiled tubing. Then liquid hydrocarbon is pumped into the annular space surrounding the jacket, causing it to swell to frictionally grip the coiled tubing. In another embodiment, metal sleeves are bonded to the cable. After installation, the coiled tubing and sleeves are crimped to each other.